MULTIJUNCTION SOLAR CELLS ON BULK GeSi SUBSTRATE

ABSTRACT

A solar cell comprising a bulk germanium silicon growth substrate; a diffused photoactive junction in the germanium silicon substrate; and a sequence of subcells grown over the substrate, with the first grown subcell either being lattice matched or lattice mis-matched to the growth substrate.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/938,246 filed Mar. 28, 2018, which is a continuation-in-part of U.S.patent application Ser. No. 15/873,135, filed Jan. 17, 2018, which inturn is a continuation-in-part of U.S. patent application Ser. No.14/828,206, filed Aug. 17, 2015.

This application is also related to U.S. patent application Ser. No.15/938,266, filed Mar. 28, 2018, which is also a continuation-in-part ofU.S. patent application Ser. No. 15/873,135, filed Jan. 17, 2018, whichin turn is a continuation-in-part of U.S. patent application Ser. No.14/828,206, filed Aug. 17, 2015.

This application is related to co-pending U.S. patent application Ser.No. 14/660,092 filed Mar. 17, 2015, which is a division of U.S. patentapplication Ser. No. 12/716,814 filed Mar. 3, 2010, now U.S. Pat. No.9,018,521; which was a continuation in part of U.S. patent applicationSer. No. 12/337,043 filed Dec. 17, 2008.

This application is also related to co-pending U.S. patent applicationSer. No. 13/872,663 filed Apr. 29, 2013, which was also acontinuation-in-part of application Ser. No. 12/337,043, filed Dec. 17,2008.

This application is also related to U.S. patent application Ser. No.14/828,197, filed Aug. 17, 2015.

All of the above related applications are incorporated herein byreference in their entireties.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to solar cells and the fabrication ofsolar cells, and more particularly the design and specification of amultijunction solar cell based on III-V semiconductor compounds grown ona bulk GeSi substrate.

Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology. Compared to silicon,III-V compound semiconductor multijunction devices have greater energyconversion efficiencies and are generally more radiation resistance,although they tend to be more complex to properly specify andmanufacture. Typical commercial III-V compound semiconductormultijunction solar cells have energy efficiencies that exceed 29.5%under one sun, air mass 0 (AM0) illumination, whereas even the mostefficient silicon technologies generally reach only about 18% efficiencyunder comparable conditions. The higher conversion efficiency of III-Vcompound semiconductor solar cells compared to silicon solar cells is inpart based on the ability to achieve spectral splitting of the incidentradiation through the use of a plurality of series connectedphotovoltaic regions with different band gap energies, and accumulatingthe voltage at a given current from each of the regions.

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as payloads useincreasing amounts of power as they become more sophisticated, andmissions and applications anticipated for five, ten, twenty or moreyears, the power-to-weight (W/kg) and power-to-area (W/m²) ratios of thesolar cell array and the lifetime efficiency of a solar cell becomesincreasingly more important. There is increasing interest not only theamount of power provided per gram of weight and spatial area of thesolar cell, not only at initial deployment but over the entire servicelife of the satellite system, or in terms of a design specification, theamount of residual power provided at the specified “end of life” (EOL),which is affected by the radiation exposure of the solar cell over timein the particular space environment of the solar cell array, the periodof the EOL being different for different missions and applications.

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures or stackedsequence of solar subcells, each subcell formed with appropriatesemiconductor layers and including a p-n photoactive junction. Eachsubcell is designed to convert photons over different spectral orwavelength bands to electrical current. After the sunlight impinges onthe front of the solar cell, and photons pass through the subcells, witheach subcell being designed for photons in a specific wavelength band.After passing through a subcell, the photons that are not absorbed andconverted to electrical energy propagate to the next subcells, wheresuch photons are intended to be captured and converted to electricalenergy.

The individual solar cells or wafers are then disposed in horizontalarrays, with the individual solar cells connected together in anelectrical series and/or parallel circuit. The shape and structure of anarray, as well as the number of cells it contains, are determined inpart by the desired output voltage and current needed by the payload orsubcomponents of the payload, the amount of electrical storage capacity(batteries) on the spacecraft, and the power demands of the payloadsduring different orbital configurations.

A solar cell designed for use in a space vehicle (such as a satellite,space station, or an interplanetary mission vehicle), has a sequence ofsubcells with compositions and band gaps which have been optimized toachieve maximum energy conversion efficiency for the AM0 solar spectrumin space. The AM0 solar spectrum in space is notably different from theAM1.5 solar spectrum at the surface of the earth, and accordinglyterrestrial solar cells are designed with subcell band gaps optimizedfor the AM1.5 solar spectrum.

There are substantially more rigorous qualification and acceptancetesting protocols used in the manufacture of space solar cells comparedto terrestrial cells, to ensure that space solar cells can operatesatisfactorily at the wide range of temperatures and temperature cyclesencountered in space. These testing protocols include (i)high-temperature thermal vacuum bake-out; (ii) thermal cycling in vacuum(TVAC) or ambient pressure nitrogen atmosphere (APTC); and in someapplications (iii) exposure to radiation equivalent to that which wouldbe experienced in the space mission, and measuring the current andvoltage produced by the cell and deriving cell performance data.

As used in this disclosure and claims, the term “space-qualified” shallmean that the electronic component (i.e., in this disclosure, the solarcell) provides satisfactory operation under the high temperature andthermal cycling test protocols. The exemplary conditions for vacuumbake-out testing include exposure to a temperature of +100° C. to +135°C. (e.g., about +100° C., +110° C., +120° C., +125° C., +135° C.) for 2hours to 24 hours, 48 hours, 72 hours, or 96 hours; and exemplaryconditions for TVAC and/or APTC testing that include cycling betweentemperature extremes of −180° C. (e.g., about −180° C., −175° C., −170°C., −165° C., −150° C., −140° C., −128° C., −110° C., −100° C., −75° C.,or −70° C.) to +145° C. (e.g., about +70° C., +80° C., +90° C., +100°C., +110° C., +120° C., +130° C., +135° C., or +145° C.) for 600 to32,000 cycles (e.g., about 600, 700, 1500, 2000, 4000, 5000, 7500,22000, 25000, or 32000 cycles), and in some space missions up to +180°C. See, for example, Fatemi et al., “Qualification and Production ofEmcore ZTJ Solar Panels for Space Missions,” Photovoltaic SpecialistsConference (PVSC), 2013 IEEE 39th (DOI: 10. 1109/PVSC 2013 6745052).Such rigorous testing and qualifications are not generally applicable toterrestrial solar cells and solar cell arrays.

Conventionally, such measurements are made for the AM0 spectrum for“one-sun” illumination, but for PV systems which use opticalconcentration elements, such measurements may be made underconcentrations such as 2×, 100×, or 1000× or more.

The space solar cells and arrays experience a variety of complexenvironments in space missions, including the vastly differentillumination levels and temperatures seen during normal earth orbitingmissions, as well as even more challenging environments for deep spacemissions, operating at different distances from the sun, such as at 0.7,1.0 and 3.0 AU (AU meaning astronomical units). The photovoltaic arraysalso endure anomalous events from space environmental conditions, andunforeseen environmental interactions during exploration missions.Hence, electron and proton radiation exposure, collisions with spacedebris, and/or normal aging in the photovoltaic array and other systemscould cause suboptimal operating conditions that degrade the overallpower system performance, and may result in failures of one or moresolar cells or array strings and consequent loss of power.

A further distinctive difference between space solar cell arrays andterrestrial solar cell arrays is that a space solar cell array utilizeswelding and not soldering to provide robust electrical interconnectionsbetween the solar cells, while terrestrial solar cell arrays typicallyutilize solder for electrical interconnections. Welding is required inspace solar cell arrays to provide the very robust electricalconnections that can withstand the wide temperature ranges andtemperature cycles encountered in space such as from −175° C. to +180°C. In contrast, solder joints are typically sufficient to survive therather narrow temperature ranges (e.g., about −40° C. to about +50° C.)encountered with terrestrial solar cell arrays.

A further distinctive difference between space solar cell arrays andterrestrial solar cell arrays is that a space solar cell array utilizessilver-plated metal material for interconnection members, whileterrestrial solar cells typically utilize copper wire for interconnects.In some embodiments, the interconnection member can be, for example, ametal plate. Useful metals include, for example, molybdenum; anickel-cobalt ferrous alloy material designed to be compatible with thethermal expansion characteristics of borosilicate glass such as thatavailable under the trade designation KOVAR from Carpenter TechnologyCorporation; a nickel iron alloy material having a uniquely lowcoefficient of thermal expansion available under the trade designationInvar, FeNi36, or 64FeNi; or the like.

An additional distinctive difference between space solar cell arrays andterrestrial solar cell arrays is that space solar cell arrays typicallyutilize an aluminum honeycomb panel for a substrate or mountingplatform. In some embodiments, the aluminum honeycomb panel may includea carbon composite face sheet adjoining the solar cell array. In someembodiments, the face sheet may have a coefficient of thermal expansion(CTE) that substantially matches the CTE of the bottom germanium (Ge)layer of the solar cell that is attached to the face sheet.Substantially matching the CTE of the face sheet with the CTE of the Gelayer of the solar cell can enable the array to withstand the widetemperature ranges encountered in space without the solar cellscracking, delaminating, or experiencing other defects. Such precautionsare generally unnecessary in terrestrial applications.

Thus, a further distinctive difference of a space solar cell from aterrestrial solar cell is that the space solar cell must include a coverglass over the semiconductor device to provide radiation resistantshielding from particles in the space environment which could damage thesemiconductor material. The cover glass is typically a ceria dopedborosilicate glass which is typically from three to six mils inthickness and attached by a transparent adhesive to the solar cell.

In summary, it is evident that the differences in design, materials, andconfigurations between a space-qualified III-V compound semiconductorsolar cell and subassemblies and arrays of such solar cells, on the onehand, and silicon solar cells or other photovoltaic devices used interrestrial applications, on the other hand, are so substantial thatprior teachings associated with silicon or other terrestrialphotovoltaic system are simply unsuitable and have no applicability tothe design configuration of space-qualified solar cells and arrays.Indeed, the design and configuration of components adapted forterrestrial use with its modest temperature ranges and cycle times oftenteach away from the highly demanding design requirements forspace-qualified solar cells and arrays and their associated components.

The assembly of individual solar cells together with electricalinterconnects and the cover glass form a so-called “CIC”(Cell-Interconnected-Cover glass) assembly, which are then typicallyelectrically connected to form an array of series-connected solar cells.The solar cells used in many arrays often have a substantial size; forexample, in the case of the single standard substantially “square” solarcell trimmed from a 100 mm wafer with cropped corners, the solar cellcan have a side length of seven cm or more.

The radiation hardness of a solar cell is defined as how well the cellperforms after exposure to the electron or proton particle radiationwhich is a characteristic of the space environment. A standard metric isthe ratio of the end of life performance (or efficiency) divided by thebeginning of life performance (EOL/BOL) of the solar cell. The EOLperformance is the cell performance parameter after exposure of thattest solar cell to a given fluence of electrons or protons (which may bedifferent for different space missions or orbits). The BOL performanceis the performance parameter prior to exposure to the particleradiation.

Charged particles in space could lead to damage to solar cellstructures, and in some cases, dangerously high voltage beingestablished across individual devices or conductors in the solar array.These large voltages can lead to catastrophic electrostatic discharging(ESD) events. Traditionally for ESD protection the backside of a solararray may be painted with a conductive coating layer to ground the arrayto the space plasma, or one may use a honeycomb patterned metal panelwhich mounts the solar cells and incidentally protects the solar cellsfrom backside radiation. Furthermore, the front side of the solar arraymay provide a conductive coating or adhesive to the periphery of thecover glass to ground the top surface of the cover glass.

The radiation hardness of the semiconductor material of the solar cellitself is primarily dependent on a solar cell's minority carrierdiffusion length (L_(min)) in the base region of the solar cell (theterm “base” region referring to the p-type base semiconductor regiondisposed directly adjacent to an n-type “emitter” semiconductor region,the boundary of which establishes the p-n photovoltaic junction). Theless degraded the parameter L_(min) is after exposure to particleradiation, the less the solar cell performance will be reduced. A numberof strategies have been used to either improve L_(min), or make thesolar cell less sensitive to L_(min) reductions. Improving L_(min) haslargely involved including a gradation in dopant elements in thesemiconductor base layer of the subcells so as to create an electricfield to direct minority carriers to the junction of the subcell,thereby effectively increasing L_(min). The effectively longer L_(min)will improve the cell performance, even after the particle radiationexposure. Making the cell less sensitive to L_(min) reductions hasinvolved increasing the optical absorption of the base layer such thatthinner layers of the base can be used to absorb the same amount ofincoming optical radiation.

Another consideration in connection with the manufacture of space solarcell arrays is that conventionally, solar cells have been arranged on asupport and interconnected using a substantial amount of manual labor.For example, first individual CICs are produced with each interconnectindividually welded to the solar cell, and each cover glass individuallymounted. Then, these CICs are connected in series to form strings,generally in a substantially manual manner, including the welding stepsfrom CIC to CIC. Then, these strings are applied to a panel substrateand electrically interconnected in a process that includes theapplication of adhesive, wiring, etc. All of this has traditionally beencarried out in a manual and substantially artisanal manner.

The energy conversion efficiency of multijunction solar cells isaffected by such factors as the number of subcells, the thickness ofeach subcell, the composition and doping of each active layer in asubcell, and the consequential band structure, electron energy levels,conduction, and absorption of each subcell, as well as the effect of itsexposure to radiation in the ambient environment over time. Theidentification and specification of such design parameters is anon-trivial engineering undertaking, and would vary depending upon thespecific space mission and customer design requirements. Since the poweroutput is a function of both the voltage and the current produced by asubcell, a simplistic view may seek to maximize both parameters in asubcell by increasing a constituent element, or the doping level, toachieve that effect. However, in reality, changing a material parameterthat increases the voltage may result in a decrease in current, andtherefore a lower power output. Such material design parameters areinterdependent and interact in complex and often unpredictable ways, andfor that reason are not “result effective” variables that those skilledin the art confronted with complex design specifications and practicaloperational considerations can easily adjust to optimize performance.

Moreover, the current (or more precisely, the short circuit currentdensity J_(sc)) and the voltage (or more precisely, the open circuitvoltage V_(oc)) are not the only factors that determine the power outputof a solar cell. In addition to the power being a function of the shortcircuit density (J_(sc)), and the open circuit voltage (V_(oc)), theoutput power is actually computed as the product of V_(oc) and J_(sc),and a Fill Factor (FF). As might be anticipated, the Fill Factorparameter is not a constant, but in fact may vary at a value between 0.5and somewhat over 0.85 for different arrangements of elementalcompositions, subcell thickness, and the dopant level and profile.Although the various electrical contributions to the Fill Factor such asseries resistance, shunt resistance, and ideality (a measure of howclosely the semiconductor diode follows the ideal diode equation) may betheoretically understood, from a practical perspective the actual FillFactor of a given subcell cannot always be predicted, and the effect ofmaking an incremental change in composition or band gap of a layer mayhave unanticipated consequences and effects on the solar subcellsemiconductor material, and therefore an unrecognized or unappreciatedeffect on the Fill Factor. Stated another way, an attempt to maximizepower by varying a composition of a subcell layer to increase the V_(oc)or J_(sc) or both of that subcell, may in fact not result in high power,since although the product V_(oc) and J_(sc) may increase, the FF maydecrease and the resulting power also decrease. Thus, the V_(oc) andJ_(sc) parameters, either alone or in combination, are not necessarily“result effective” variables that those skilled in the art confrontedwith complex design specifications and practical operationalconsiderations can easily adjust to optimize performance.

Furthermore, the fact that the short circuit current density (J_(sc)),the open circuit voltage (V_(oc)), and the fill factor (FF), areaffected by the slightest change in such design variables, the purity orquality of the chemical pre-cursors, or the specific process flow andfabrication equipment used, and such considerations further complicatesthe proper specification of design parameters and predicting theefficiency of a proposed design which may appear “on paper” to beadvantageous.

It must be further emphasized that in addition to process and equipmentvariability, the “fine tuning” of minute changes in the composition,band gaps, thickness, and doping of every layer in the arrangement hascritical effect on electrical properties such as the open circuitvoltage (V_(oc)) and ultimately on the power output and efficiency ofthe solar cell.

To illustrate the practical effect, consider a design change thatresults in a small change in the V_(oc) of an active layer in the amountof 0.01 volts, for example changing the V_(oc) from 2.72 to 2.73 volts.Assuming all else is equal and does not change, such a relatively smallincremental increase in voltage would typically result in an increase ofsolar cell efficiency from 29.73% to 29.84% for a triple junction solarcell, which would be regarded as a substantial and significantimprovement that would justify implementation of such design change.

For a single junction GaAs subcell in a triple junction device, a changein V_(oc) from 1.00 to 1.01 volts (everything else being the same) wouldincrease the efficiency of that junction from 10.29% to 10.39%, about a1% relative increase. If it were a single junction stand-alone solarcell, the efficiency would go from 20.58% to 20.78%, still about a 1%relative improvement in efficiency.

Present day commercial production processes are able to define andestablish band gap values of epitaxially deposited layers as preciselyas 0.01 eV, so such “fine tuning” of compositions and consequential opencircuit voltage results are well within the range of operationalproduction specifications for commercial products.

Another important mechanical or structural consideration in the choiceof semiconductor layers for a solar cell is the desirability of theadjacent layers of semiconductor materials in the solar cell, i.e. eachlayer of crystalline semiconductor material that is deposited and grownto form a solar subcell, have similar or substantially similar crystallattice constants or parameters.

Here again there are trade-offs between including specific elements inthe composition of a layer which may result in improved voltageassociated with such subcell and therefore potentially a greater poweroutput, and deviation from exact crystal lattice matching with adjoininglayers as a consequence of including such elements in the layer whichmay result in a higher probability of defects, and therefore lowermanufacturing yield.

In that connection, it should be noted that there is no strictdefinition of what is understood to mean two adjacent layers are“lattice matched” or “lattice mismatched”. For purposes in thisdisclosure, “lattice mismatched” refers to two adjacently disposedmaterials or layers (with thicknesses of greater than 100 nm) havingin-plane lattice constants of the materials in their fully relaxed statediffering from one another by less than 0.02% in lattice constant.(Applicant notes that this definition is considerably more stringentthan that proposed, for example, in U.S. Pat. No. 8,962,993, whichsuggests less than 0.6% lattice constant difference as defining “latticemismatched” layers).

The conventional wisdom for many years has been that in a monolithictandem solar cell, “ . . . the desired optical transparency and currentconductivity between the top and bottom cells . . . would be bestachieved by lattice matching the top cell material to the bottom cellmaterial. Mismatches in the lattice constants create defects ordislocations in the crystal lattice where recombination centers canoccur to cause the loss of photogenerated minority carriers, thussignificantly degrading the photovoltaic quality of the device. Morespecifically, such effects will decrease the open circuit voltage(V_(oc)), short circuit current (J_(sc)), and fill factor (FF), whichrepresents the relationship or balance between current and voltage foreffective output” (Jerry M. Olson, U.S. Pat. No. 4,667,059, “Current andLattice Matched Tandem Solar Cell”).

As progress has been made toward higher efficiency multijunction solarcells with four or more subcells, nevertheless, “it is conventionallyassumed that substantially lattice-matched designs are desirable becausethey have proven reliability and because they use less semiconductormaterial than metamorphic solar cells, which require relatively thickbuffer layers to accommodate differences in the lattice constants of thevarious materials” (Rebecca Elizabeth Jones-Albertus et al., U.S. Pat.No. 8,962,993).

As discussed in Applicant's U.S. Pat. No. 7,339,109, the currentstate-of-the-art triple junction solar cell is a device that uses layersof indium gallium phosphide (InGaP), gallium arsenide (GaAs), andgermanium (Ge). The contribution of a germanium (Ge) junction improvesthe energy conversion efficiency of a solar cell by adding open-circuitvoltage to the structure. More recently, there is interest in the designof alternative semiconductor structures with higher band gaps thangermanium and greater open-circuit voltages than that of germanium foruse in the bottom subcell.

In addition to the use of a different growth substrate, the presentdisclosure further proposes design features for metamorphicmultijunction solar cells which departs from such conventional wisdomfor increasing the efficiency of the multijunction solar cell inconverting solar energy (or photons) to electrical energy and optimizingsuch efficiency at the “end-of-life” period.

SUMMARY OF THE DISCLOSURE Objects of the Disclosure

It is an object of the present disclosure to provide in a multijunctionsolar cell grown on a bulk GeSi substrate.

It is another object of the present invention to provide an uprightmetamorphic four junction solar cell in which the average band gap ofall four subcells is greater than 1.35 eV grown on a GeSi substrate.

It is another object of the present invention to increase the band gapof the bottom subcell in a multijunction solar cell by using a GeSisubstrate in lieu of germanium.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingobjects.

Features of the Invention

All ranges of numerical parameters set forth in this disclosure are tobe understood to encompass any and all subranges or “intermediategeneralizations” subsumed therein. For example, a stated range of “1.0to 2.0 eV” for a band gap value should be considered to include any andall subranges beginning with a minimum value of 1.0 eV or more andending with a maximum value of 2.0 eV or less, e.g., 1.0 to 1.2, or 1.3to 1.4, or 1.5 to 1.9 eV.

Briefly, and in general terms, the present disclosure provides afour-junction space-qualified solar cell assembly designed for operationat AM0 and at a 1 MeV electron equivalent fluence of at least 1×10¹⁴e/cm², the solar cell comprising subcells, wherein a combination ofcompositions and band gaps of the subcells is designed to maximizeefficiency of the solar cell at a predetermined time, after initialdeployment, when the solar cell is deployed in space at AM0 and at anoperational temperature in the range of 40 to 70 degrees Centigrade, thepredetermined time being at least five years and referred to as theend-of-life (EOL), the solar cell comprising: providing an upper firstsolar subcell composed of indium gallium aluminum phosphide and having afirst band gap in the range of 2.0 to 2.2 eV; providing a second solarsubcell adjacent to said upper first solar subcell and including anemitter layer composed of indium gallium phosphide or aluminum indiumgallium arsenide, and a base layer composed of aluminum indium galliumarsenide and having a second band gap in the range of approximately 1.55to 1.8 eV and being lattice matched with the upper first solar subcell,wherein the emitter and base layers of the second solar subcell form aphotoelectric junction; providing a third solar subcell adjacent to saidsecond solar subcell and composed of indium gallium arsenide and havinga third band gap less than that of the second solar subcell and beinglattice matched with the second solar subcell; and providing a fourthsolar subcell adjacent to said third solar subcell and composed ofgermanium silicon and has an indirect band gap in the range of 0.7 to1.1 eV, or 0.85 to 1.05 eV; wherein each of the upper first solarsubcell, the second solar subcell and the third solar subcell islattice-mismatched to the fourth solar subcell, and a numerical sum ofthe band gaps of the four solar subcells, divided by four) is equal to1.35 eV.

In another aspect, the present disclosure provides a method for forminga four-junction space-qualified solar cell assembly designed foroperation at AM0 and at a 1 MeV electron equivalent fluence of at least1×10¹⁴ e/cm², the solar cell comprising subcells, wherein a combinationof compositions and band gaps of the subcells is designed to maximizeefficiency of the solar cell at a predetermined time, after initialdeployment, when the solar cell is deployed in space at AM0 and at anoperational temperature in the range of 40 to 70 degrees Centigrade, thepredetermined time being at least five years and referred to as theend-of-life (EOL), the solar cell comprising: providing an upper firstsolar subcell composed of indium gallium aluminum phosphide and having afirst band gap in the range of 2.0 to 2.2 eV; providing a second solarsubcell adjacent to said upper first solar subcell and including anemitter layer composed of indium gallium phosphide or aluminum indiumgallium arsenide, and a base layer composed of aluminum indium galliumarsenide and having a second band gap in the range of approximately 1.55to 1.8 eV and being lattice matched with the upper first solar subcell,wherein the emitter and base layers of the second solar subcell form aphotoelectric junction; providing a third solar subcell adjacent to saidsecond solar subcell and composed of indium gallium arsenide and havinga third band gap less than that of the second solar subcell and beinglattice matched with the second solar subcell; and providing a fourthsolar subcell adjacent to said third solar subcell and composed ofgermanium silicon and has an indirect band gap in the range of 0.7 to1.1 eV, or 0.85 to 1.05 eV; wherein each of the upper first solarsubcell, the second solar subcell and the third solar subcell islattice-mismatched to the fourth solar subcell, and wherein a numericalsum of the band gaps of the four solar subcells, divided by four, isequal to 1.35 eV.

In another aspect, the present disclosure provides a method ofmanufacturing a multijunction solar cell comprising providing a growthsubstrate, forming a first solar subcell in the growth substrate havinga band gap in the range of 0.83 to 0.88 eV, growing a sequence of layersof semiconductor material using a disposition process to form a solarcell comprising a plurality of subcells including a first middle subcelldisposed over the growth substrate and having a band gap in the range of0.9 to 1.6 eV, at least a second middle subcell disposed over the firstmiddle subcell and having a band gap in the range of approximately 1.55to 1.8 eV and an upper subcell disposed over the last middle subcell anda band gap in the range of 2.0 to 2.20 eV; wherein the growth substrateis composed of SiGe with the Ge content in the SiGe in the range of 85%to 97%.

In another aspect, the present disclosure provides a method ofmanufacturing a multijunction solar cell comprising: providing a growthsubstrate, forming a first solar subcell in the growth substrate,growing a sequence of layers of semiconductor material using adisposition process to form a solar cell comprising a plurality ofsubcells including a first middle subcell disposed over and latticematched with respect to the growth substrate and having a band gap inthe range of 0.9 to 1.6 eV, at least a second middle subcell disposedover the first middle subcell; and an upper subcell disposed over thelast middle subcell and a band gap in the range of 2.0 to 2.20 eV;wherein the growth substrate is composed of SiGe with the Ge content inthe SiGe in the range of 85% to 97%.

In another aspect, the present disclosure provides a method ofmanufacturing a multijunction solar cell comprising: providing a growthsubstrate, forming a first solar subcell in the growth substrate, andgrowing a sequence of layers of semiconductor material using adisposition process to form a solar cell comprising a plurality ofsubcells including a first middle subcell disposed over and latticemis-matched with respect to the growth substrate and having a band gapin the range of 0.9 to 1.6 eV, at least a second middle subcell disposedover the first middle subcell and having a band gap in the range ofapproximately 1.55 to 1.8 eV and an upper subcell disposed over the lastmiddle subcell and a band gap in the range of 2.0 to 2.20 eV; whereinthe growth substrate is composed of SiGe with the Ge content in the SiGesubstrate in the range of 85% to 97%.

In some embodiments, the bulk germanium silicon substrate is grown bythe Czochralski method, and the Ge content in the SiGe substrate in therange of 85% to 87%.

In another aspect, the present disclosure provides a method ofmanufacturing a multijunction solar cell comprising: growing a growthsubstrate by the Czochralski method; forming a first solar subcell inthe growth substrate; and growing a sequence of layers of semiconductormaterial using a deposition process to form a solar cell comprising aplurality of subcells over the growth substrate; wherein the growthsubstrate is composed of SiGe with the Ge content by mole fraction inthe SiGe substrate being in the range of 85% to 97%.

In some embodiments, the germanium silicon growth substrate has anindirect band gap in the range of 0.7 to 1.1 eV, or 0.85 to 1.05 eV.

In some embodiments, the germanium silicon substrate has a thickness inthe range of 50 to 600 μm, or 100 to 200 μm.

In some embodiments, there further comprises a buffer layer and/ornucleation layer disposed directly over the growth substrate andcomposed of a material that has a similar lattice parameter as thegrowth substrate.

In some embodiments, the nucleation layer comprises InGaP.

In some embodiments, the buffer layer comprises GaAs.

In some embodiments, a graded interlayer is provided above the growthsubstrate and the buffer layer which is compositionally graded tolattice match the growth substrate on one side and the directly adjacentmiddle solar subcell on the other side, and is composed of any of the A,S, P, N, Sb based III-V compound semiconductors subject to theconstraints of having its in-plane lattice parameter throughout itsthickness being greater than or equal to that of the growth substrate.

In some embodiments, the upper first solar subcell has a band gap ofapproximately 2.05 eV, the second solar subcell has a band gap ofapproximately 1.55 eV; and the third solar subcell has a band gap in therange of 0.9 to 1.55 eV.

In some embodiments, the third solar subcell has a band gap of 1.41 eVor less.

In some embodiments, the third solar subcell has a band gap in the rangeof 1.15 to 1.35 eV.

In some embodiments, the third solar subcell has a band gap in the rangeof 1.1 to 1.2 eV.

In some embodiments, the third solar subcell has a band gap ofapproximately 1.2 eV.

In some embodiments, the fourth solar subcell has a band gap in therange of 0.83 to 0.88 eV as measured at 300 degrees Kelvin,corresponding to a percentage of Si in the GeSi substrate ranging from13% to 15%.

In some embodiments, the third solar subcell is lattice mis-matched withrespect to the fourth solar subcell.

In some embodiments, the third solar subcell is lattice matched withrespect to the fourth solar subcell.

In some embodiments, the upper first subcell is composed of indiumgallium aluminum phosphide; the second solar subcell includes an emitterlayer composed of indium gallium phosphide or aluminum indium galliumarsenide, and a base layer composed of aluminum indium gallium arsenide;the third solar subcell is composed of indium gallium arsenide; and thefourth subcell is composed of germanium silicon.

In some embodiments, there further comprises an intermediate layer abovethe bottom subcell or growth substrate wherein the intermediate layer iscompositionally graded to lattice match the third solar subcell on oneside and the fourth solar subcell on the other side and is composed ofany of the As, P, N, Sb based III-V compound semiconductors subject tothe constraints of having the in-plane lattice parameter greater than orequal to that of the third solar subcell and less than or equal to thatof the lower fourth solar subcell, and having a band gap energy greaterthan that of the fourth solar subcell.

In some embodiments, the intermediate layer is compositionallystep-graded with between one and four steps to lattice match the fourthsolar subcell on one side and composed of In_(x)Ga_(1-x)As or(In_(x)Ga_(1-x))_(y)Al_(1-y)As with 0<x<1, 0<y<1, and x and y selectedsuch that the band gap remains in the range of 1.15 to 1.41 eVthroughout its thickness.

In some embodiments, the intermediate layer has a constant band gap inthe range of 1.15 to 1.41 eV, or 1.2 to 1.35 eV, or 1.25 to 1.30 eV.

In some embodiments, there further comprises a distributed Braggreflector (DBR) layer adjacent to and between the third and the fourthsolar subcells and arranged so that light can enter and pass through thethird solar subcell and at least a portion of which can be reflectedback into the third solar subcell by the DBR layer.

In some embodiments, the distributed Bragg reflector layer is composedof a plurality of alternating layers of lattice matched materials withdiscontinuities in their respective indices of refraction.

In some embodiments, the difference in refractive indices betweenalternating layers is maximized in order to minimize the number ofperiods required to achieve a given reflectivity, and the thickness andrefractive index of each period determines the stop band and itslimiting wavelength.

In some embodiments, the DBR layer includes a first DBR layer composedof a plurality of p type In_(z)Al_(x)Ga_(1-x-z)As layers, and a secondDBR layer disposed over the first DBR layer and composed of a pluralityof p type In_(w)Al_(y)Ga_(1-y-x)As layers, where 0<x<1, 0<y<1, 0<z<1 andy is greater than x.

In some embodiments, the selection of the composition of the subcellsand their band gaps maximizes the efficiency at high temperature (in therange of 40 to 70 degrees Centigrade) in deployment in space at apredetermined time after the initial deployment (referred to as thebeginning-of-life or (BOL), such predetermined time being referred to asthe end-of-life (EOL) and being at least five years, and the averageband gap (i.e., the numerical average of the lowest band gap material ineach subcell) of all four subcells is greater than 1.35 eV.

In some embodiments, at least one of the upper sublayers of the gradedinterlayer has a larger lattice constant than the adjacent layers to theupper sublayer disposed above the grading interlayer.

In some embodiments, the difference in lattice constant between theadjacent third and fourth subcells is in the range of 0.1 to 0.2Angstroms.

In some embodiments, there further comprises at least a first threadingdislocation inhibition layer having a thickness in the range of 0.10 to1.0 microns and disposed over said second solar subcell.

In some embodiments, there further comprises at least a second threadingdislocation inhibition layer having a thickness in the range of 0.10 to1.0 micron and composed of InGa(Al)P, the second threading dislocationinhibition layer being disposed over and directly adjacent to saidgrading interlayer for reducing the propagation of threadingdislocations, said second threading dislocation inhibition layer havinga composition different from a composition of the first threadingdislocation inhibition layer.

In some embodiments, additional layer(s) may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingsummaries.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present disclosure. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

Additional aspects, advantages, and novel features of the presentdisclosure will become apparent to those skilled in the art from thisdisclosure, including the following detailed description as well as bypractice of the disclosure. While the disclosure is described below withreference to preferred embodiments, it should be understood that thedisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalapplications, modifications and embodiments in other fields, which arewithin the scope of the disclosure as disclosed and claimed herein andwith respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better and more fully appreciated by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a graph representing the band gap of GeSi and their latticeconstants;

FIG. 2A is a cross-sectional view of a bulk GeSi substrate;

FIG. 2B is a cross-sectional view of an embodiment of a GeSi solar cellafter several stages of fabrication including the deposition of certainsemiconductor layers on the growth substrate of FIG. 2A, according tothe present disclosure;

FIG. 2C is a cross-sectional view of an embodiment of a GeSi solar cellof FIG. 2B after the diffusion of dopant elements into the growthsubstrate, according to the present disclosure; and

FIG. 3 is a highly simplified cross-sectional view of a portion of anembodiment of a multijunction solar cell grown on a GeSi substrateaccording to the present disclosure.

GLOSSARY OF TERMS

“III-V compound semiconductor” refers to a compound semiconductor formedusing at least one elements from group III of the periodic table and atleast one element from group V of the periodic table. III-V compoundsemiconductors include binary, tertiary and quaternary compounds. GroupIII includes boron (B), aluminum (Al), gallium (Ga), indium (In) andthallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb) and bismuth (Bi).

“Band gap” refers to an energy difference (e.g., in electron volts (eV))separating the top of the valence band and the bottom of the conductionband of a semiconductor material. More particularly, the expression“band gap” of a solar subcell, which internally has layers of differentband gaps shall be defined to mean the band gap of the layer of thesolar subcell in which the majority of the charge carriers are generated(such sublayer typically being the p-type base semiconductor layer ofthe base/emitter photovoltaic junction of such subcell). In the eventsuch layer in turn has sublayers with different band gaps (such as thecase of a base layer having a graded composition and more particularly agraded band gap), the sublayer of that solar subcell with the lowestband gap shall be taken as defining the “band gap” of such a subcell.Apart from a solar subcell, and more generally in the case of aspecifically designated semiconductor region (such as a metamorphiclayer), in which that semiconductor region has sublayers or subregionswith different band gaps (such as the case of a semiconductor regionhaving a graded composition and more particularly a graded band gap),the sublayer or subregion of that semiconductor region with the lowestband gap shall be taken as defining the “band gap” of that semiconductorregion.

“Beginning of Life (BOL)” refers to the time at which a photovoltaicpower system is initially deployed in operation.

“Bottom subcell” refers to the subcell in a multijunction solar cellwhich is furthest from the primary light source for the solar cell.

“Compound semiconductor” refers to a semiconductor formed using two ormore chemical elements.

“Current density” refers to the short circuit current density Jscthrough a solar subcell through a given planar area, or volume, ofsemiconductor material constituting the solar subcell.

“Deposited”, with respect to a layer of semiconductor material, refersto a layer of material which is epitaxially grown over anothersemiconductor layer.

“Dopant” refers to a trace impurity element that is contained within asemiconductor material to affect the electrical or opticalcharacteristics of that material. As used in the context of the presentdisclosure, typical dopant levels in semiconductor materials are in the1016 to 1019 atoms per cubic centimeter range. The standard notation ornomenclature, when a particular identified dopant is proscribed, is touse, for example, the expression “GaAs:Se” or “GaAs:C” for selenium orcarbon doped gallium arsenide respectively. Whenever a ternary orquaternary compound semiconductor is expressed as “AlGaAs” or “GaInAsP”,it is understood that all three or four of the constituent elements aremuch higher in mole concentration, say on the 1% level or above, whichis in the 1021 atoms/cm-3 or larger range. Such constituent elements arenot considered “dopants” by those skilled in the art since the atoms ofthe constituent element form part of the crystal structure of thecompound semiconductor. In addition, a further distinction is that adopant has a different valence number than the constituent componentelements. In a commonly implemented III-V compound semiconductor such asAlGaInAs, none of the individual elements Al, Ga, In, or As areconsidered to be dopants since they have the same valence as thecomponent atoms that make up the crystal lattice.

“End of Life (EOL)” refers to a predetermined time or times after theBeginning of Life, during which the photovoltaic power system has beendeployed and has been operational. The EOL time or times may, forexample, be specified by the customer as part of the required technicalperformance specifications of the photovoltaic power system to allow thesolar cell designer to define the solar cell subcells and sublayercompositions of the solar cell to meet the technical performancerequirement at the specified time or times, in addition to other designobjectives. The terminology “EOL” is not meant to suggest that thephotovoltaic power system is not operational or does not produce powerafter the EOL time.

“Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

“Inverted metamorphic multijunction solar cell” or “IMM solar cell”refers to a solar cell in which the subcells are deposited or grown on asubstrate in a “reverse” sequence such that the higher band gapsubcells, which would normally be the “top” subcells facing the solarradiation in the final deployment configuration, are deposited or grownon a growth substrate prior to depositing or growing the lower band gapsubcells.

“Layer” refers to a relatively planar sheet or thickness ofsemiconductor or other material. The layer may be deposited or grown,e.g., by epitaxial or other techniques.

“Lattice mismatched” refers to two adjacently disposed materials orlayers (with thicknesses of greater than 100 nm) having in-plane latticeconstants of the materials in their fully relaxed state differing fromone another by less than 0.02% in lattice constant. (Applicant expresslyadopts this definition for the purpose of this disclosure, and notesthat this definition is considerably more stringent than that proposed,for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6%lattice constant difference).

“Metamorphic layer” or “graded interlayer” refers to a layer thatachieves a gradual transition in lattice constant generally throughoutits thickness in a semiconductor structure.

“Middle subcell” refers to a subcell in a multijunction solar cell whichis neither a Top Subcell (as defined herein) nor a Bottom Subcell (asdefined herein).

“Short circuit current (I_(sc))” refers to the amount of electricalcurrent through a solar cell or solar subcell when the voltage acrossthe solar cell is zero volts, as represented and measured, for example,in units of milliamps.

“Short circuit current density”—see “current density”.

“Solar cell” refers to an electronic device operable to convert theenergy of light directly into electricity by the photovoltaic effect.

“Solar cell assembly” refers to two or more solar cell subassembliesinterconnected electrically with one another.

“Solar cell subassembly” refers to a stacked sequence of layersincluding one or more solar subcells.

“Solar subcell” refers to a stacked sequence of layers including a p-nphotoactive junction composed of semiconductor materials. A solarsubcell is designed to convert photons over different spectral orwavelength bands to electrical current.

“Space qualified” refers to an electronic component (e.g., as used inthis disclosure, to a solar cell) provides satisfactory operation underthe high temperature and thermal cycling test protocols that establishtypical “qualification” requirements for use by customers who utilizesuch components in the outer space environment. The exemplary conditionsfor such qualifications include (i) vacuum bake-out testing thatincludes exposure to a temperature of +100° C. to +135° C. (e.g., about+100° C., +110° C., +120° C., +125° C., +135° C.) for 2 hours to 24hours, 48 hours, 72 hours, or 96 hours; and (ii) TVAC and/or APTC testthat includes cycling between temperature extremes of −180° C. (e.g.,about −180° C., −175° C., −170° C., −165° C., −150° C., −140° C., −128°C., −110° C., −100° C., −75° C., or −70° C.) to +145° C. (e.g., about+70° C., +80° C., +90° C., +100° C., +110° C., +120° C., +130° C., +135°C., or +145° C.) for 600 to 32,000 cycles (e.g., about 600, 700, 1500,2000, 4000, 5000, 7500, 22000, 25000, or 32000 cycles), and in somespace missions up to +180° C. See, for example, Fatemi et al.,“Qualification and Production of Emcore ZTJ Solar Panels for SpaceMissions,” Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th(DOI: 10.1109/PVSC 2013 6745052).

“Substantially current matched” refers to the short circuit currentthrough adjacent solar subcells being substantially identical (i.e.within plus or minus 1%).

“Top subcell” or “upper subcell” refers to the subcell in amultijunction solar cell which is closest to the primary light sourcefor the solar cell.

“Upright multijunction solar cell” refers to a solar cell in which thesubcells are deposited or grown on a substrate in a sequence such thatthe lower band gap subcells are deposited or grown on a growth substrateprior to depositing or growing the higher band gap subcells.

“ZTJ” refers to the product designation of a commercially availableSolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

A variety of different features of multijunction solar cells (as well asinverted metamorphic multijunction solar cells) are disclosed in therelated applications noted above. Some, many or all of such features maybe included in the structures and processes associated with the“upright” solar cells of the present disclosure. However, moreparticularly, some embodiments of the present disclosure are directed tothe fabrication of a multijunction solar cell grown on a germaniumsilicon growth substrate.

The novel proposal for the use of germanium silicon instead of germaniumas the growth substrate and bottom solar subcell is the cornerstone ofthe present disclosure. Following on Applicant's earlier advances asrepresented by U.S. Pat. No. 7,339,109 and subsequent proposals anddevelopments, the formation of a photoelectric junction in such agermanium silicon substrate is an improvement that extends the spectralband that can be captured by the bottom subcell in a multijunction solarcell to be an indirect band gap in the range of 0.7 to 1.1 eV. Inaddition to the specification of a germanium silicon bottom subcell, thepresent disclosure also provides for an embodiment of a multijunctionsolar cell in which the two lower subcells (e.g., the third and fourthsubcells in a four junction solar cell) are lattice mismatched. Morespecifically, in some embodiments, the present disclosure relates tofour junction solar cells with direct band gaps in the range of 2.0 to2.15 eV (or higher) for the top subcell, and (i) 1.65 to 1.8 eV, and(ii) 1.41 eV or less, for the middle subcells, and (iii) 0.7 to 1.1 eVindirect bandgaps for the bottom subcell, respectively.

The present disclosure, similar to the related applications ofApplicant, provides an unconventional four junction design (with threegrown lattice matched subcells, which are lattice mismatched to the GeSisubstrate) that leads to a surprising significant performanceimprovement over that of traditional three junction solar cell on Gedespite the substantial current mismatch present between the top threejunctions and the bottom Ge junction. This performance gain isespecially realized at high temperature and after high exposure to spaceradiation by the proposal of incorporating high band gap semiconductorsthat are inherently more resistant to radiation and temperature, thusspecifically addressing the problem of ensuring continues adequateefficiency and power output at the “end-of-life”.

In some embodiments, the fourth subcell is germanium silicon, while inother embodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs,or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN.InGaSbBiN or other III-V or II-VI compound semiconductor material.

The indirect band gap of germanium at room temperature is about 0.66 eV,while the direct band gap of germanium at room temperature is 0.8 eV.Those skilled in the art will normally refer to the “band gap” ofgermanium as 0.67 eV, since it is lower than the direct band gap valueof 0.8 eV.

In the present disclosure, the indirect band gap of the germaniumsilicon growth substrate would broadly be in the range of 0.7 to 1.1 eV,or for certain applications in the range of 0.85 to 1.05 eV.

More specifically, the present disclosure intends to provide arelatively simple and reproducible technique for “upright” processing ofmetamorphic multijunction solar cells, that is suitable for use in ahigh volume production environment in which various semiconductor layersare grown on a growth substrate in an MOCVD reactor, and subsequentprocessing steps are defined and selected to minimize the physicaldamage to the quality of the deposited layers, thereby simplifying waferhandling and ensuring a relatively high yield of operable solar cellsmeeting specifications at the conclusion of the fabrication processes.

As suggested above, incremental improvements in the design ofmultijunction solar cells are made in view of a variety of new spacemissions and application requirements. Moreover, although suchimprovements may be relatively minute quantitative modifications in thecomposition, lattice constant, or band gap of certain subcells oradjoining layers, as we noted above, such minute parametric changes canprovide substantial improvements in efficiency that specifically addressthe “problems” that have been identified associated with the existingcurrent commercial multijunction solar cells, and provide a “solution”that represents an “inventive step” in the design process.

Prior to discussing the specific embodiments of the present disclosure,a brief discussion of some of the issues associated with the design ofmultijunction solar cells in the context of the composition ordeposition of various specific layers in embodiments of the product asspecified and defined by Applicant is in order.

There are a multitude of properties that should be considered inspecifying and selecting the composition of, inter alia, a specificsemiconductor layer, the back metal layer, the adhesive or bondingmaterial, or the composition of the supporting material for mounting asolar cell thereon. For example, some of the properties that should beconsidered when selecting a particular layer or material are electricalproperties (e.g. conductivity), optical properties (e.g., band gap,absorbance and reflectance), structural properties (e.g., thickness,strength, flexibility, Young's modulus, etc.), chemical properties(e.g., growth rates, the “sticking coefficient” or ability of one layerto adhere to another, stability of dopants and constituent materialswith respect to adjacent layers and subsequent processes, etc.), thermalproperties (e.g., thermal stability under temperature changes,coefficient of thermal expansion), and manufacturability (e.g.,availability of materials, process complexity, process variability andtolerances, reproducibility of results over high volume, reliability andquality control issues).

In view of the trade-offs among these properties, it is not alwaysevident that the selection of a material based on one of itscharacteristic properties is always or typically “the best” or “optimum”from a commercial standpoint or for Applicant's purposes. For example,theoretical studies may suggest the use of a quaternary material with acertain band gap for a particular subcell would be the optimum choicefor that subcell layer based on fundamental semiconductor physics. As anexample, the teachings of academic papers and related proposals for thedesign of very high efficiency (over 40%) solar cells may thereforesuggest that a solar cell designer specify the use of a quaternarymaterial (e.g., InGaAsP) for the active layer of a subcell. A few suchdevices may actually be fabricated by other researchers, efficiencymeasurements made, and the results published as an example of theability of such researchers to advance the progress of science byincreasing the demonstrated efficiency of a compound semiconductormultijunction solar cell. Although such experiments and publications areof “academic” interest, from the practical perspective of the Applicantsin designing a compound semiconductor multijunction solar cell to beproduced in high volume at reasonable cost and subject to manufacturingtolerances and variability inherent in the production processes, such an“optimum” design from an academic perspective is not necessarily themost desirable design in practice, and the teachings of such studiesmore likely than not point in the wrong direction and lead away from theproper design direction. Stated another way, such references mayactually “teach away” from Applicant's research efforts and the ultimatesolar cell design proposed by the Applicants.

In view of the foregoing, it is further evident that the identificationof one particular constituent element (e.g. indium, or aluminum) in aparticular subcell, or the thickness, band gap, doping, or othercharacteristic of the incorporation of that material in a particularsubcell, is not a “result effective variable” that one skilled in theart can simply specify and incrementally adjust to a particular leveland thereby increase the efficiency of a solar cell. The efficiency of asolar cell is not a simple linear algebraic equation as a function ofthe amount of gallium or aluminum or other element in a particularlayer. The growth of each of the epitaxial layers of a solar cell in areactor is a non-equilibrium thermodynamic process with dynamicallychanging spatial and temporal boundary conditions that is not readily orpredictably modeled. The formulation and solution of the relevantsimultaneous partial differential equations covering such processes arenot within the ambit of those of ordinary skill in the art in the fieldof solar cell design.

Even when it is known that particular variables have an impact onelectrical, optical, chemical, thermal or other characteristics, thenature of the impact often cannot be predicted with much accuracy,particularly when the variables interact in complex ways, leading tounexpected results and unintended consequences. Thus, significant trialand error, which may include the fabrication and evaluative testing ofmany prototype devices, often over a period of time of months if notyears, is required to determine whether a proposed structure with layersof particular compositions, actually will operate as intended, let alonewhether it can be fabricated in a reproducible high volume manner withinthe manufacturing tolerances and variability inherent in the productionprocess, and necessary for the design of a commercially viable device.

Furthermore, as in the case here, where multiple design variablesinteract in unpredictable ways, the proper choice of the combination ofvariables can produce new and unexpected results, and constitute an“inventive step”.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a depositionmethod, such as Molecular Beam Epitaxy (MBE), Organo Metallic VaporPhase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD),or other vapor deposition methods for the growth may enable the layersin the monolithic semiconductor structure forming the cell to be grownwith the required thickness, elemental composition, dopant concentrationand grading and conductivity type, and are within the scope of thepresent disclosure.

The present disclosure is in one embodiment directed to a growth processusing a metal organic chemical vapor deposition (MOCVD) process in astandard, commercially available reactor suitable for high volumeproduction. Other embodiments may use other growth technique, such asMBE. More particularly, regardless of the growth technique, the presentdisclosure is directed to the materials and fabrication steps that areparticularly suitable for producing commercially viable multijunctionsolar cells or inverted metamorphic multijunction solar cells usingcommercially available equipment and established high-volume fabricationprocesses, as contrasted with merely academic expositions of laboratoryor experimental results.

Some comments about MOCVD processes used in one embodiment are in orderhere.

It should be noted that the layers of a certain target composition in asemiconductor structure grown in an MOCVD process are inherentlyphysically different than the layers of an identical target compositiongrown by another process, e.g. Molecular Beam Epitaxy (MBE). Thematerial quality (i.e., morphology, stoichiometry, number and locationof lattice traps, impurities, and other lattice defects) of an epitaxiallayer in a semiconductor structure is different depending upon theprocess used to grow the layer, as well as the process parametersassociated with the growth. MOCVD is inherently a chemical reactionprocess, while MBE is a physical deposition process. The chemicals usedin the MOCVD process are present in the MOCVD reactor and interact withthe wafers in the reactor, and affect the composition, doping, and otherphysical, optical and electrical characteristics of the material. Forexample, the precursor gases used in an MOCVD reactor (e.g. hydrogen)are incorporated into the resulting processed wafer material, and havecertain identifiable electro-optical consequences which are moreadvantageous in certain specific applications of the semiconductorstructure, such as in photoelectric conversion in structures designed assolar cells. Such high order effects of processing technology do resultin relatively minute but actually observable differences in the materialquality grown or deposited according to one process technique comparedto another. Thus, devices fabricated at least in part using an MOCVDreactor or using a MOCVD process have inherent different physicalmaterial characteristics, which may have an advantageous effect over theidentical target material deposited using alternative processes.

FIG. 1 is a graph representing the band gap of GeSi and their latticeconstants in the range of interest of the present disclosure, i.e, a Geconcentration by mole fraction of over 85%, and in some embodiments inthe range of 85% to 87%.

FIG. 2A is a cross-sectional view of a GeSi growth substrate 100 whichas bulk semiconductor material may be between 50 and 600 microns inthickness, or in some embodiments between 100 and 200 microns.

FIG. 2B depicts the epitaxial growth of a nucleation layer 102 and abuffer layer 103 on top of the GeSi growth substrate 100. The bufferlayer 103 may be composed of GaAs.

FIG. 2C depicts the result of diffusion of As and P into the growthsubstrate 100, and the formation of a photovoltaic junction depicted bythe dashed line in the interior of the substrate 100. As a result of thediffusion, the upper portion 101 of the substrate 100 is converted intoan n+ type semiconductor which forms the emitter of the solar subcellformed in the growth substrate 100, which in the embodiments discussedbelow will be the “bottom” subcell D of a multijunction solar cell.

Turning to a multijunction solar cell device of the present disclosure,FIG. 3 is a cross-sectional view of an embodiment of a four junctionsolar cell 450 after several stages of fabrication including the growthof certain semiconductor layers on the growth substrate up to thecontact layer 322 according to the present disclosure.

As shown in the illustrated example of FIG. 4, the bottom or fourthsubcell D includes a growth substrate 300 formed of p-type germaniumsilicon (“GeSi”) which also serves as a base layer. A back metal contactpad 350 formed on the bottom of base layer 300 provides electricalcontact to the multijunction solar cell 200. The bottom subcell D,further includes, for example, an n+ type nucleation and buffer layer302. The nucleation layer is deposited over the growth substrate, andthe emitter layer is formed in the substrate by diffusion of dopantsinto the GeSi substrate, thereby forming the n+ type GeSi layer 301.Heavily doped p-type aluminum indium gallium arsenide (“AlGaAs”) andheavily doped n-type gallium arsenide (“GaAs”) tunneling junction layers304, 303 may be deposited over the buffer layer to provide a lowresistance pathway between the bottom and middle subcells.

In the embodiment depicted, an intermediate graded interlayer 506,comprising in one embodiment step-graded sublayers 505 a through 505 zz,is disposed over the tunnel diode layer 303/304. In particular, thegraded interlayer provides a transition in the in-plane lattice constantfrom the lattice constant of the substrate subcell D to the largerlattice constant of the middle and upper subcells C, B and A.

The graph on the left side of FIG. 3 depicts the in-plane latticeconstant being incrementally monotonically increased from sublayer 505 athrough sublayer 505 zz, such sublayers being fully relaxed.

At least a first “alpha” or threading dislocation inhibition layer 504,preferably composed of p-type InGaP, is deposited over the tunnel diode303/304, to a thickness of from 0.10 to about 1.0 micron. Such an alphalayer is intended to prevent threading dislocations from propagating,either opposite to the direction of growth into the bottom subcell D, orin the direction of growth into the subcell C, and is more particularlydescribed in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeldet al.). More generally, the alpha layer has a different compositionthan the adjacent layers above and below it.

The metamorphic layer (or graded interlayer) 506 is deposited over thealpha layer 504 using a surfactant. Layer 505 is preferably acompositionally step-graded series of p-type InGaAs or InGaAlAs layers,preferably with monotonically changing lattice constant, so as toachieve a gradual transition in lattice constant in the semiconductorstructure from subcell D to subcell C while minimizing threadingdislocations from occurring. In one embodiment, the band gap of layer506 is constant throughout its thickness, preferably approximately equalto 1.22 to 1.34 eV, or otherwise consistent with a value slightlygreater than the band gap of the middle subcell C. In anotherembodiment, the band gap of the sublayers of layer 506 vary in the rangeof 1.22 to 1.34 eV, with the first layer having a relatively high bandgap, and subsequent layers incrementally lower band gaps. One embodimentof the graded interlayer may also be expressed as being composed ofInxGal-xAs, with 0<x<1, 0<y<1, and x and y selected such that the bandgap of the interlayer remains constant at approximately 1.22 to 1.34 eVor other appropriate band gap.

In one embodiment, aluminum is added to one sublayer to make oneparticular sublayer harder than another, thereby forcing dislocations inthe softer material.

In the surfactant assisted growth of the metamorphic layer 506, asuitable chemical element is introduced into the reactor during thegrowth of layer 506 to improve the surface characteristics of the layer.In the preferred embodiment, such element may be a dopant or donor atomsuch as selenium (Se) or tellurium (Te). Small amounts of Se or Te aretherefore incorporated in the metamorphic layer 506, and remain in thefinished solar cell. Although Se or Te are the preferred n-type dopantatoms, other non-isoelectronic surfactants may be used as well.

Surfactant assisted growth results in a much smoother or planarizedsurface. Since the surface topography affects the bulk properties of thesemiconductor material as it grows and the layer becomes thicker, theuse of the surfactants minimizes threading dislocations in the activeregions, and therefore improves overall solar cell efficiency.

As an alternative to the use of non-isoelectronic one may use anisoelectronic surfactant. The term “isoelectronic” refers to surfactantssuch as antimony (Sb) or bismuth (Bi), since such elements have the samenumber of valence electrons as the P atom of InGaP, or the As atom inInGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactantswill not typically be incorporated into the metamorphic layer 506.

In one embodiment of the present disclosure, the layer 506 is composedof a plurality of layers of InGaAs, with monotonically changing latticeconstant, each layer having a band gap in the range of 1.22 to 1.34 eV.In some embodiments, the band gap is constant in the range of 1.27 to1.31 eV through the thickness of layer 505. In some embodiments, theconstant band gap is in the range of 1.28 to 1.29 eV.

The advantage of utilizing a constant bandgap material such as InGaAs isthat arsenide-based semiconductor material is much easier to process instandard commercial MOCVD reactors.

Although the described embodiment of the present disclosure utilizes aplurality of layers of InGaAs for the metamorphic layer 506 for reasonsof manufacturability and radiation transparency, other embodiments ofthe present disclosure may utilize different material systems to achievea change in lattice constant from subcell C to subcell D. Otherembodiments of the present disclosure may utilize continuously graded,as opposed to step graded, materials. More generally, the gradedinterlayer may be composed of any of the As, P, N, Sb based III-Vcompound semiconductors subject to the constraints of having thein-plane lattice parameter less than or equal to that of the third solarsubcell C and greater than or equal to that of the fourth solar subcellD. In some embodiments, the layer 505 has a band gap energy greater thanthat of the third solar subcell C, and in other embodiments has a bandgap energy level less than that of the third solar subcell C.

In some embodiments, a second “alpha” or threading dislocationinhibition layer 507, preferably composed of p type GaInP, is depositedover metamorphic buffer layer 506, to a thickness of from 0.10 to about1.0 micron. Such an alpha layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the fourth subcell D, or in the direction of growth into thethird subcell C, and is more particularly described in U.S. PatentApplication Pub. No. 2009/0078309 A1 (Cornfeld et al.).

In the specific embodiment depicted in FIG. 3, the top or uppermostsublayer 505 zz of the graded interlayer 506 is strained or onlypartially relaxed, and has a lattice constant which is greater than thatof the layer above it, i.e., the alpha layer 507 (should there be asecond alpha layer) or the BSF layer 306. In short, in this embodiment,there is an “overshoot” of the last one sublayer 505 zz of the gradingsublayers, as depicted on the left hand side of FIG. 4B, which shows thestep-grading of the lattice constant becoming larger from layer 505 a to505 zz, and then decreasing back to the lattice constant of the upperlayers 507 through 322.

In the illustrated example of FIG. 3, the third subcell C includes ahighly doped p-type aluminum indium gallium arsenide (“AlInGaAs”) backsurface field (“BSF”) layer 306, a p-type InGaAs base layer 307, ahighly doped n-type indium gallium arsenide (“InGaAs”) emitter layer 308and a highly doped n-type indium aluminum phosphide (“AlInP₂”) or indiumgallium phosphide (“GaInP”) window layer 309. The InGaAs base layer 307of the subcell C can include, for example, approximately 1.5% In. Othercompositions may be used as well. The base layer 307 is formed over theBSF layer 306 after the BSF layer is deposited over the DBR layers 305.

The window layer 309 is deposited on the emitter layer 308 of thesubcell C. The window layer 309 in the subcell C also helps reduce therecombination loss and improves passivation of the cell surface of theunderlying junctions. Before depositing the layers of the subcell B,heavily doped n-type InGaP and p-type AlGaAs (or other suitablecompositions) tunneling junction layers 310, 311 may be deposited overthe subcell C.

The second subcell B includes a highly doped p-type aluminum indiumgallium arsenide (“AlInGaAs”) back surface field (“BSF”) layer 312, ap-type AlInGaAs base layer 313, a highly doped n-type indium galliumphosphide (“InGaP₂”) or AlInGaAs layer 314 and a highly doped n-typeindium gallium aluminum phosphide (“AlGaAlP”) window layer 315. TheInGaP emitter layer 314 of the subcell B can include, for example,approximately 50% In. Other compositions may be used as well.

Before depositing the layers of the top or upper first cell A, heavilydoped n-type InGaP and p-type AlGaAs tunneling junction layers 316, 317may be deposited over the subcell B.

In the illustrated example, the top subcell A includes a highly dopedp-type indium aluminum phosphide (“InAlP₂”) BSF layer 318, a p-typeInGaAlP base layer 319, a highly doped n-type InGaAlP emitter layer 320and a highly doped n-type InAlP₂ window layer 321. The base layer 319 ofthe top subcell A is deposited over the BSF layer 318 after the BSFlayer 318 is formed.

After the cap or contact layer 322 is deposited, the grid lines areformed via evaporation and lithographically patterned and deposited overthe cap or contact layer 322.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofstructures or constructions differing from the types of structures orconstructions described above.

Although described embodiments of the present disclosure utilizes avertical stack of four subcells, various aspects and features of thepresent disclosure can apply to stacks with fewer or greater number ofsubcells, i.e. two junction cells, three junction cells, five, six,seven junction cells, etc.

In addition, although the disclosed embodiments are configured with topand bottom electrical contacts, the subcells may alternatively becontacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure mayutilize an arrangement of one or more, or all, homojunction cells orsubcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor both of whichhave the same chemical composition and the same band gap, differing onlyin the dopant species and types, and one or more heterojunction cells orsubcells. Subcell C, with p-type and n-type InGaAs is one example of ahomojunction subcell.

In some cells, a thin so-called “intrinsic layer” may be placed betweenthe emitter layer and base layer, with the same or different compositionfrom either the emitter or the base layer. The intrinsic layer mayfunction to suppress minority-carrier recombination in the space-chargeregion. Similarly, either the base layer or the emitter layer may alsobe intrinsic or not-intentionally-doped (“NID”) over part or all of itsthickness.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP,AlGaInAs, AlGaInPAs, GaInP, GalnAs, GaInPAs, AlGaAs, AlInAs, AlInPAs,GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GalnSb, AlGaInSb, AlN, GaN, InN,GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials,and still fall within the spirit of the present invention.

While the solar cell described in the present disclosure has beenillustrated and described as embodied in a conventional uprightmultijunction solar cell, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Thus, while the description of the semiconductor device described in thepresent disclosure has focused primarily on solar cells or photovoltaicdevices, persons skilled in the art know that other optoelectronicdevices, such as thermophotovoltaic (TPV) cells, photodetectors andlight-emitting diodes (LEDS), are very similar in structure, physics,and materials to photovoltaic devices with some minor variations indoping and the minority carrier lifetime. For example, photodetectorscan be the same materials and structures as the photovoltaic devicesdescribed above, but perhaps more lightly-doped for sensitivity ratherthan power production. On the other hand LEDs can also be made withsimilar structures and materials, but perhaps more heavily-doped toshorten recombination time, thus radiative lifetime to produce lightinstead of power. Therefore, this invention also applies tophotodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, from the foregoing others can, by applyingcurrent knowledge, readily adapt the present invention for variousapplications. Such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

1. A method of manufacturing a multijunction solar cell comprising:providing a growth substrate; forming a first solar subcell in thegrowth substrate; growing a sequence of layers of semiconductor materialusing a disposition process to form a solar cell comprising a pluralityof subcells including a first middle subcell disposed over the growthsubstrate and having a band gap in the range of 0.9 to 1.6 eV, at leasta second middle subcell disposed over the first middle subcell andhaving a band gap in the range of approximately 1.55 to 1.8 eV and anupper subcell disposed over the last middle subcell and a band gap inthe range of 2.0 to 2.20 eV; wherein the growth substrate is composed ofGeSi with the Ge content in the GeSi substrate in the range of 85% to87%.
 2. A method as defined in claim 1, wherein the first solar subcellhas a band gap of less than 2.15 eV, the second middle solar subcell hasa band gap of less than 1.73 eV; and the first middle solar subcell hasa band gap in the range of 1.15 to 1.2 eV.
 3. A method as defined inclaim 1, wherein the first solar subcell has a band gap of 2.05 eV, andthe first solar subcell has an indirect band gap of 0.7 to 1.1 eV, or0.85 to 1.65 eV.
 4. A method as defined in claim 1, wherein the band gapof the first middle solar subcell is less than 1.41 eV, and greater thanthat of the first solar subcell.
 5. A method as defined in claim 1,further comprising: providing a distributed Bragg reflector (DBR) layeradjacent to and disposed between the first middle and the first solarsubcells and arranged so that light can enter and pass through the firstmiddle solar subcell and at least a portion of which can be reflectedback into the first middle solar subcell by the DBR layer, and iscomposed of a plurality of alternating sublayers of lattice matchedmaterials with discontinuities in their respective indices ofrefraction; and wherein the difference in refractive indices betweenalternating sublayers is maximized in order to minimize the number ofperiods required to achieve a given reflectivity, and the thickness andrefractive index of each period determines the stop band and itslimiting wavelength.
 6. A method as defined in claim 5, wherein the DBRlayer includes a first DBR layer composed of a plurality of p typeIn_(z)Al_(x)Ga_(1-x-z)As sublayers, and a second DBR layer disposed overand adjacent to the first DBR layer and composed of a plurality of ptype In_(w)Al_(y)Ga_(1-y-w)As sublayers, where 0<w<1, 0<y<1, 0<z<1 and yis greater than x, thereby increasing the reflection bandwidth of theDBR layer.
 7. A method as defined in claim 1, wherein the growthsubstrate is lattice mismatched with respect to the first middlesubcell, and has a band gap between 0.83 and 0.88 eV as measured at 300degrees Kelvin, corresponding to a percentage of Si in the GeSisubstrate ranging between 13.0 and 15.0 percent by mole fraction.
 8. Amethod as defined in claim 1, wherein the first subcell is composed of abase layer of (In_(x)Ga_(1-x))_(1-y)Al_(y)P where x is 0.505, and y is0.142, corresponding to a band gap of 2.10 eV, and an emitter layer of(In_(x)Ga_(1-x))_(1-y)Al_(y)P where x is 0.505, and y is 0.107,corresponding to a band gap of 2.05 eV.
 9. A method as defined in claim1, further comprising a tunnel diode disposed over the growth substrate,and an intermediate layer disposed between the first middle subcell andthe tunnel diode, wherein the intermediate layer is compositionallygraded to lattice match the first middle solar subcell on one side andthe tunnel diode on the other side and is composed of any of the As, P,N, Sb based III-V compound semiconductors subject to the constraints ofhaving the in-plane lattice parameter greater than or equal to that ofthe first middle solar subcell and different from that of the tunneldiode, and having a band gap energy greater than that of the growthsubstrate.
 10. A method as defined in claim 1, further comprising anintermediate layer disposed between the first middle subcell and thegrowth substrate wherein the intermediate layer is compositionallystep-graded with between one and four steps to lattice match the growthsubstrate on one side and composed of In_(x)Ga_(1-x)As or(In_(x)Ga_(1-x))_(y)Al_(1-y)As with 0<x<1, 0<y<1, and x and y selectedsuch that the band gap is in the range of 1.15 to 1.41 eV throughout itsthickness.
 11. A method as defined in claim 10, wherein the intermediatelayer has a graded band gap in the range of 1.15 to 1.41 eV, or 1.2 to1.35 eV, or 1.25 to 1.30 eV.
 12. A method as defined in claim 1, whereineither (i) the emitter layer; or (ii) the base layer and emitter layer,or the upper subcell have different lattice constants from the latticeconstant of the second middle subcell.
 13. A method as defined in claim1, further comprising: providing a distributed Bragg reflector (DBR)layer adjacent to and beneath the first middle solar subcell andarranged so that light can enter and pass through the first middle solarsubcell and at least a portion of which can be reflected back into thefirst middle solar subcell by the DBR layer, wherein the distributedBragg reflector layer is composed of a plurality of alternating layersof lattice matched materials with discontinuities in their respectiveindices of refraction, wherein the difference in refractive indicesbetween alternating layers is maximized in order to minimize the numberof periods required to achieve a given reflectivity, and the thicknessand refractive index of each period determines the stop band and itslimiting wavelength, and wherein the DBR layer includes a first DBRlayer composed of a plurality of p type In_(z)Al_(x)Ga_(1-x-z)As layers,and a second DBR layer disposed over the first DBR layer and composed ofa plurality of p type In_(w)Al_(y)Ga_(1-y-w)As layers, where 0<w<1,0<x<1, 0<y<1, 0<z<1 and y is greater than x; and providing anintermediate layer disposed between the DBR layer and the first solarsubcell, wherein the intermediate layer is compositionally step-gradedto lattice match the DBR layer on one side and the first solar subcellon the other side, and is composed of any of the As, P, N, Sb basedIII-V compound semiconductors subject to the constraints of having thein-plane lattice parameter greater than or equal to that of the DBRlayer and less than or equal to that of the first solar subcell, andhaving a band gap energy greater than that of the first solar subcell.14. A method as defined in claim 1, wherein each subcell includes anemitter region and a base region, and one or more of the subcells have abase region having a gradation in doping that increases exponentiallyfrom 1×10¹⁵ atoms per cubic centimeter adjacent the p-n junction to4×10¹⁸ atoms per cubic centimeter adjacent to the adjoining layer at therear of the base, and an emitter region having a gradation in dopingthat decreases from approximately 5×10¹⁸ atoms per cubic centimeter inthe region immediately adjacent the adjoining layer to 5×10¹⁷ atoms percubic centimeter in the region adjacent to the p-n junction.
 15. Amethod as defined in claim 9, wherein at least one of the uppersublayers of the intermediate layer has a larger lattice constant thanthe adjacent layers of the upper sublayer disposed directly above theintermediate layer.
 16. A method as defined in claim 1, wherein thedifference in lattice constant between the adjacent first middle subcelland the first subcell is in the range of 0.1 to 0.2 Angstroms.
 17. Amethod as defined in claim 1, further comprising an inactive majoritycarrier layer (i.e., a window, BSF, or tunnel diode layer) disposed overthe first middle subcell or second middle solar subcell, and having alattice constant that is greater than that of the first middle subcelland the first subcell so that the tunnel diode layers are strained intension.
 18. A method as defined in claim 1, further comprising a firstthreading dislocation inhibition layer having a thickness in the rangeof 0.10 to 1.0 microns disposed over said second middle solar subcell.19. A method as defined in claim 18, further comprising a secondthreading dislocation inhibition layer having a thickness in the rangeof 0.10 to 1.0 micron and composed of InGa(Al)P, the second threadingdislocation inhibition layer being disposed over and directly adjacentto said grading interlayer for reducing the propagation of threadingdislocations, said second threading dislocation inhibition layer havinga composition different from a composition of the first threadingdislocation inhibition layer.
 20. A method as defined in claim 1,including providing an intermediate layer disposed between the firstmiddle solar subcell and the first solar subcell so as to provide agradual transition in lattice constant in semiconductor structure fromthe first middle solar subcell to the first solar subcell, wherein theintermediate layer has a band gap that in constant throughout itsthickness, and wherein the multijunction solar cell is a four junctionsolar cell in which the numerical sum of the band gaps of the four solarsubcells, divided by four, is equal to 1.35 eV.